Microporous membranes made of polyolefins are commonly used, for example, for separation and selective permeation of various substances and as separating members. Specifically, they are used, for example, as microfiltration membranes, separators for fuel cell or condenser, matrices for functional membranes for packing a functional material in pores to impart a novel function, and separators for cell. When used for such purposes, they are suitably used, in particular, as separators for lithium ion cells for notebook-sized personal computers, mobile phones, digital cameras and the like. The reason is, for example, that they have not only characteristics such as mechanical strength and permeability but also pore-clogging properties and heat resistance.
The term “pore-clogging properties” used here means the following capability: when the interior of a cell is overheated by overcharge or the like, the microporous membrane is melted to clog its pores, so that the cell reaction is stopped, resulting in assurance of the safety of the cell. It is considered that the effect on the safety is increased with a lowering of a temperature at which the pores are clogged. The term “heat resistance” used here means a capability to retain a shape for the maintenance of insulation between electrodes even at a high temperature. The microporous membrane is required to have a low thermal shrinkage stress and to be unbreakable, at a high temperature.
In patent documents 1 to 3, the present applicant has proposed a microporous membrane improved in pore-clogging properties by employment of low-melting polyethylenes such as copolymerized polyethylenes and low-density polyethylenes as some or all of components. When such a method is adopted, the pore-clogging properties are improved but the heat resistance is liable to be deteriorated.
In patent documents 4 to 6, microporous membranes containing wax are proposed. When such a method is adopted, the pore-clogging properties are improved but the homogeneity of a starting material for the microporous membrane is decreased, so that the quality of the membrane tends to be lowered, for example, non-molten matters remain in the membrane. In addition, it is presumed that when such a method is adopted, the heat resistance is unavoidably deteriorated.
In patent documents 7 to 9 and in patent documents 10 to 12 by the present applicant, there have been proposed microporous membranes obtained by using a two-stage polymerization polyethylene alone or a blend of a single-stage polymerization polyethylene having a high molecular weight and a single-stage polymerization polyethylene having a low molecular weight. In the former method, the employment of the two-stage polymerization polyethylene permits discharge at a high polymer concentration. But the microporous membrane is insufficient in heat resistance and moreover, its degree of thermal shrinkage is presumed to be high because of insufficient thermal fixation. On the other hand, in the latter method, the polyethylene component having a high molecular weight is effective in improving the heat resistance, while the polyethylene component having a low molecular weight is effective in improving the pore-clogging properties. Therefore, such a method makes it possible to improve the pore-clogging properties and the heat resistance at the same time to a certain degree. Such a tendency toward the improvement of the above-mentioned performance characteristics at the same time by this method, however, is an undesirable tendency for the quality of the membrane because the low-molecular weight component and the high-molecular weight component tend to separate from each other, resulting in low homogeneity of a starting material for the microporous membrane.
In addition, in patent documents 13 and 14, microporous membranes comprising a high-molecular weight polyethylene, a low-molecular weight polyethylene, a polypropylene and a low-melting polyethylene have been proposed. Such a method is also effective in improving the pore-clogging properties. It, however, is not desirable for the quality of the membrane because the thermal shrinkage stress at a high temperature is increased by the low-melting component and a starting material for the membrane is difficult to make homogeneous. Moreover, the membrane tends to have a high thermal shrinkage stress at a high temperature because it does not contain a sufficient amount of the low-molecular weight component.    Patent document 1: Japanese Patent No. 3113287    Patent document 2: JP-A-2003-217554    Patent document 3: JP-A-2003-231772    Patent document 4: JP-A-8-20659    Patent document 5: JP-A-10-17702    Patent document 6: JP-A-11-106533    Patent document 7: Japanese Patent No. 2657431    Patent document 8: Japanese Patent No. 3009495    Patent document 9: JP-A-11-92587    Patent document 10: Japanese Patent No. 2794179    Patent document 11: Japanese Patent No. 3305006    Patent document 12: Japanese Patent No. 3258737    Patent document 13: JP-A-2001-72788    Patent document 14: JP-A-2001-72792